This invention relates to a fast switching driver for changing the current through an inductive load, and more particularly to a driver for an inductor of a phase shifting device.
Phased array antenna systems are well known, in which the beam can be steered by varying the relative phase shift between the elements of the array. The use of Fox phase shifters as RF phase control elements is quite common in phased array antennas. A book by M. I. Skolnik, "Introduction to Radar Systems", McGraw Hill Book Co., 1962, describes array antennas in section 7.7, pages 294-320, phase shifting devices for such arrays on pages 307-311, and the Fox device in particular on pages 308-309, with references on page 351 to articles by A. G. Fox et al in Proc. IRE, December 1947, and Bell System Tech J, January 1955. The Fox phase shifter is based on the properties of circularly polarized waves in round waveguide. It consists of three sections of round waveguide, e.g., two quarter and one half wave plate. When a linear polarized wave is injected at one end the RF signal is changed from vertical to circular polarization at the input to the half wave plate. The output of that plate changes the right hand circular polarization to a left hand circular polarization. That wave is then converted again to a linear polarized wave at the output of the second quarter wave plate. When the half wave plate is rotated mechanically or by electrical means, a phase shift occurs. The original Fox phase shifter was a mechanical type. Later versions use sine/cosine coils, which rotate a quadrapole magnetic field. This rotation causes the half wave plate to rotate electronically. The driver for the electronic control is the subject of the present invention.
The driver has equal application for line stretcher types of phase shifters as employed in microwave circulators. In this type of phase shifter the apparent electrical length of the waveguide is changed by controlling the magnetic field transversing ferrite slabs attached to the inner walls of the waveguide.
The significant aspect of these phase shift devices with respect to the invention is that they constitute an inductive load for the driver, in which the current must be rapidly switched from one level to another or reversed in direction, with a relatively long steady state condition between the switching intervals. There are many devices other than phase shifters which provide similar inductive loads for current drivers.
Present drivers for these phase shifters or other inductive loads make use of a sufficiently high voltage applied via a device such as a transistor so that the required rate of change of current may be achieved. This causes high driver losses. The dissipation in the driver is equal to the difference between the transient and steady state voltages multiplied by the inductor current. When very fast change is required, the driver dissipation may be ten times more than that of the inductor coil.
Another way to minimize the switching time of inductive phase shift drivers for a magnetic circuit of given reluctance, is to decrease the number of turns. This is because the inductance is directly proportional to the square of the number of turns. This approach is limited by phase shifter coil geometry and the added current requirement placed on driver transistors.